1. Field of the Invention
The present invention relates to the field of mode-switching transformers (BALUN) having the function of converting a common mode signal into a differential mode signal and, more specifically, such transformers intended to be associated with a low-pass filter.
2. Discussion of the Related Art
FIG. 1 very schematically shows in the form of blocks a first example of association of a mode-switching transformer 1 (BALUN) and of low-pass filters 2 and 3 (LPF). In this example, filters 2 and 3 are connected to the differential outputs of balun 1 having its common-mode input connected to an antenna 4. This is a typical example of application of balun transformers used in portable phones. They are then intended to convert the signals received on the antenna to make them exploitable by the circuits internal to the telephone.
FIG. 2 shows a second conventional example of assembly associating a low-pass filter 5 with a balun transformer 1. In this example, the low-pass filter is placed upstream of transformer 1, that is, between antenna 4 and the common-mode input of balun 1.
FIG. 3 shows the equivalent electric diagram of a balun transformer 1. On the common-mode side, two inductive elements 11 and 12 are series-connected between a common-mode access UNB and a port left open O. On the differential mode side, two inductive elements 13 and 14 are series-connected between the two differential-mode accesses BAL1 and BAL2. The junction point of inductances 13 and 14 is grounded.
Elements 11 to 14 are sized according to the central frequency of the passband desired for the balun, each inductive winding 11 to 14 having a length equal to one quarter of the wavelength of this central frequency.
FIG. 4 shows a conventional example of an equivalent electric diagram of a low-pass filter 2, 3, or 4. Such a filter is generally symmetrical and comprises two input-output terminals 15 and 16 between which are connected a first capacitor 17 in parallel with an inductance 18, each terminal 15 and 16 being further grounded by a capacitor, respectively, 19 or 20.
The filter elements are sized according to the cut-off frequency desired for the filter, this frequency being selected according to the passband of transformer 1.
The fact that the two mode-switching elements (low-pass filter and balun) are separated from each other adversely affects the circuit integration compactness, and thus the miniaturization.